Chrissie Wellington, sitting cross-legged on a sofa, does not look like an iron woman on a raw winter afternoon. We've just walked in from a dark and windy park on the outskirts of London and, in a warm house filled by her laughter and animated conversation, the former civil servant seems endearingly ordinary. It's easy to remember that, exactly five years ago, just before she became a professional triathlete in late February 2007 and the most remarkable woman in sport, Wellington still worked for the British government as an adviser in international development with particular expertise in Nepal.

Yet, for all her friendly charm and supple intelligence, it soon becomes obvious that Wellington, who turned 34 last week, is ferociously determined. She describes what would seem like unbearable suffering to most of us as "discomfort" when coolly listing the demands of an Ironman race – in which an athlete completes a 2.4 mile swim and a 112 mile bike ride before running a marathon of 26.2 miles.

Wellington, who has won every single one of her 13 Ironman competitions, holds the women's world record of 8hr 18min 32sec – which is 32 minutes faster than the previous record which had stood for 14 years. Perhaps the simplest way to explain her extraordinary endurance is to underline the fact that, immediately after her draining ordeals in the water and on the bike, Wellington can run a marathon in 2 hours and 44 minutes.

She has won four world championships – but Wellington is even more impressive when revealing the corrosive insecurities of her past. "The times and number of victories I've managed matter less than the continual surprise I feel. It's so empowering to defy your own perceptions of what is possible and to keep on opening doors within yourself that you didn't know even existed. Looking back five years ago this month I would never have predicted anything that has since happened."

Wellington looks at me with an even gaze as, more softly this time, she repeats one word again: "Never."

And then, after a long pause, she opens up the most deeply personal facets of her complex personality to the wider world. "We all have talents that, sometimes, we never quite fulfil. We're all scared, deep down, but maybe we just need to lay it on the line and explore our abilities and just not be afraid of failing. But I know it's hard."

Wellington is about to publish a riveting and often startling biography and the imminent exposure taps into these more complicated emotions. "I'm such a contradictory character" she says. "I can be confident in my ability but at the same time, I can lack so much confidence. I guess I exemplified that when we walked back from the park and you said you liked the book. That made me so happy. I crave that. It's not enough for me to be proud of what I've achieved. I still want the approval of others."

That human need once became, for Wellington, dangerously consuming. "I haven't revealed it before," she says, taking a small breath before talking about her past eating disorders. "There is still a stigma and so I swing between wanting to talk about it and being ashamed because to me it's still tantamount to weakness. But every week I receive letters from women and young girls. They don't know I've suffered from eating disorders but they explain how they're suffering from these same afflictions. I've got a message in my inbox now from an American girl saying: 'I don't know where to turn, can you help me?' She's a triathlete."

Wellington looks moved, if distressed, until she straightens her shoulders and says she is ready for her life to change once people understand the depths from which she emerged. "When I first started in sport I did it for purely selfish reasons – to be as good as I could be. I still have that desire but the more I do it the more I realise I can do something else. Hopefully, I can show people what is possible."

She enjoyed a comfortable middle-class childhood in Norfolk and then achieved a first-class honours degree in geography. And yet, with no knowledge of the sporting greatness within her, Wellington was so beset by insecurity that she went out of her way to change her body-shape – and ended up in the grip of anorexia and bulimia.

"The victims of such illnesses are often very ambitious, outwardly successful young women who pursue these ideas of control and achievement," she says. "We're driven, compulsive, obsessive, competitive, persistent and seek perfection. That can be channelled incredibly negatively."

Wellington was saved by her family. "My parents' reaction got to me," she says, "especially my dad. He is less outwardly emotional but when I saw he was near tears looking at me I understood how worried they were. But it was really my brother who made me face the truth. His tough love really worked."

She has now reached the point where, without vanity, she can talk about "loving" her own body. Rather than focusing on her outward appearance, Wellington can celebrate all her body has achieved – in conjunction with an exceptional mental strength. "Sport has given me a different perception and respect for my body. I cannot judge its success by whether I can fit into a pair of skinny jeans. I look at other women and, yes, they're beautiful with stereotypically wonderful figures but I can think, 'Yeah, but you haven't won what I've won.' I might not be perfect but I can love my body now."

Wellington came to the tortuous world of Ironman as an amateur jogger – who discovered by chance that she loved running. She ran her first marathon at the age of 25, in London in 2002, and surprised herself by finishing in 3hr 8 sec. Five years later, she decided to quit her government job and become a triathlete. Wellington chose to work with the world's most successful but controversial triathlon coach.

Brett Sutton's infamy centres on the fact that he once had a sexual relationship with a 15-year-old girl he trained. In later years, as an adult, she made a formal complaint and Sutton was charged. He argued that the affair had been consensual but the judge still accused Sutton of "abusing [his] role [as Australia's national triathlon coach] to an inexcusable degree." Sutton received a suspended two-year sentence.

"It was unforgiveable at the time," Wellington says of Sutton's behaviour. "But Brett repented. He said to me, 'I despise myself for the suffering others have gone through for what I did.' Some will choose never to forgive him. Others do. But Brett is not the cold mercenary that is usually depicted. He has an incredible emotional intelligence.

"On the first Monday we started working together we went for a swim and he said, 'I can see you've suffered from an eating disorder.' I thought: 'How the bloody hell does this paunchy little man know?' He can read me like a book and developed an understanding very quickly of my personality – my impatience, my inability to rest my mind, my desire for perfection. He summed me up in one sentence. You want to be different, yet you crave to be normal. It's true. I didn't want to be like the other girls. I wanted to be better than them. But I wanted to be one of them."

In some of the book's most gripping pages, Wellington details how she and Sutton fought for control. Recounting his demands that Wellington, a self-confessed "control-freak", become "supine like a slave", there are vague echoes of a sado-masochistic relationship. Does such a description sound overblown to her?

"No," Wellington says. "Some of the problems I had, the worries and fears in the first six months with Brett, were due to the fact I didn't trust him. I was questioning him constantly and that's so draining. It was only when I allowed him to take control that I could feel that sense of liberation."

Sutton shouted at Wellington and treated her cruelly in training. How could she bring herself to trust him? "Away from training he had a warm, caring and sensitive side. But he could be cold and callous and in front of the other athletes he was very harsh. He was either going to make me or break me."

Wellington emits a bleak little laugh. "Those thoughts went through my head when he shouted at me that, 'You're weak, you'll never be successful.' Well, yeah, I was crying into my goggles but the other part of me said, 'I'll show you, you bastard. I'm going to prove to you that I'm not weak.' I needed that but it was so liberating handing the reigns to Brett. It was definitely right at that point and it made me the athlete I am. Could I do it now? Probably not because Brett taught me most of it anyway so I don't need him now – but I've also built on what he taught me and learnt from others.

"Brett was hugely influential in my life. No other coach would have done what he did, in 2007, when he threw a rookie who had never done any long-distance triathlons, into the world championships in Kona. Brett had seen something special in me. He knew long before anyone else that I could do it."

Wellington shocked the Ironman world by winning – and beating her nearest rival by five minutes. Yet her triumph last year, in the same world championships, transcended all expectations. She began the race with a damaged pectoral muscle which she'd torn two weeks earlier. She was grazed and bruised and had been to hospital just a few days before the race. Wellington was told it would be a miracle if she even finished.

She was forced to swim more slowly than usual and, also struggling on her bike, she began the marathon 21 minutes behind the leader. Wellington scrunches up her face. "Normally, I've not suffered much more than discomfort. Maybe my discomfort is someone else's debilitating pain. But, this time, I got it. This was bad pain. Persistent and deep, deep pain."

Wellington still ran the marathon in 2 hours 52 minutes and 41 seconds – and won by almost three minutes. "Around Christmas last year," she remembers, "Brett called me. He hasn't trained me for years but he said, 'You used to doubt if you could win a race under duress. You wondered if you would crumble. Now you know. You've got nothing more to prove.' It's true. That's why I have maybe completed the journey. I'm not retiring but I'm going to take a break for a year or two."

Wellington will work on her charities and in the media, and possibly train exclusively for next year's London marathon where she believes she could run "2.20 something".

But Wellington will also sit back, for once, and savour a new truth. "I'm content," she says as late afternoon gives way to a black and freezing evening. "I'm happy. I've got nothing to prove to anyone anymore – not even to myself."

A Life Without Limits by Chrissie Wellington is published by Constable on Thursday, priced £18.99